The major aim of the proposed experiments is to define the mechanism by which estrogen and progesterone control the initiation of implantation. Blastocysts from normal and delayed implanting pregnancies will be compared in vivo and in vitro for: rates of CO2 evolution, glycogen content, dry weight, lipid content, mitotic rates, protein content and amino acid uptake and incorporation. The effect of estrogen and progesterone on each of these indices will be measured in an attempt to determine the steps involved in the initiation of implantation. The techniques of egg transplantation, in vitro culture, radioautrography and scintillation counting, quantitative determinations of glycogen (enzymatic) and protein (chemical), microbalance (Lowry fishpole) determinations of dry weight and flat mount nuclei counts will be used in the proposed experiments. The potential significance of this research is that it may clarify the roles of the maternal organism, the endometrium, and the blastocyst in implantation and add to our understanding of normal and abnormal development.